1. Field of the Invention
The present invention relates to heat exchangers and to solar energy collectors and in particular to a method for fabricating solar energy collector panels of the flat-plate type.
2. Prior Art
An urgent, current need exists for alternative energy supplies to fill the gap between industrial, commercial and residential energy needs and the ability of dwindling supplies of fossil fuels to satisfy those needs. Nuclear fission offers near-term relief, but supplies of fissionable material are also limited and, in addition, nuclear energy production is accompanied by as yet unsolved problems of waste disposal, plant and fuel transportation security risks, and high capital costs.
Since a large amount of energy produced is used for heating homes, and commercial and industrial buildings, and for heating water supplies for bathing, washing and similar activities, elimination or reduction of the energy required for those activities from the current energy distribution channels would mean that current energy sources can be stretched over a longer period of time and used in ways for which they are most efficient; i.e., petroleum reserves could be applied to transportation needs while coal and uranium reserves could be used to produce electricity for use in lighting, industrial processes, communications and similar areas for which electricity is ultimately required.
It has been proposed to use the sun's energy for this purpose, and indeed solar energy is being used on a limited basis for residential heating and has been so used for quite some period of time. More widespread use is required for this alternative to produce any appreciable reduction in overall energy needs, but widespread use on a residential basis is generally hampered by the high capital costs of collection and storage apparatus since the cost must generally be bourne by the individual homeowner. Although it may be shown that savings in other costs over a period of time will equal or exceed the cost of purchasing and installing solar collector equipment, the large capital outlay requirement is nonetheless a serious deterrent to widespread use of solar energy.
Of the proposed solar energy collection schemes, the most commonly used and, generally speaking, the most convenient and efficient is the flat-plate collector. Although more exotic schemes are available, the flat-plate collector is generally considered best for reasons of initial cost and compatibility with existing architectural styles, and compatibility with existing home systems.
Of the many flat-plate collectors designed, most employ a thin sheet of copper to which is bonded a copper tubing which meanders over the surface of the sheet to achieve maximum thermal conductivity between all areas of the sheet and the walls of the tubing. Common additions include darkening of the surface of the sheet to achieve higher energy absorption, and, insulation of the back of the collector plate and glazing of the front to prevent excessive heat loss.
A drawback of the flat-plate collector, and of many other engineering situations as well, lies in the oft-encountered dilemma that if the device works efficiently it is expensive to produce, while if it is inexpensively produced, it works inefficiently.
The primary cost of the solar collector lies in its raw materials, but substantial labor costs are encountered and must be reckoned with in the manufacturing process. Of these, the cost associated with achievement of a close thermal bond between the copper tubing and the copper sheet loom large. At the same time, failure to devote sufficient attention to thermal bonding will result in an inefficient collector design since less than maximum transfer of heat energy will occur between the sheet and the tubing.
The most common solution is to position the copper tubing against the copper sheet and then to solder, braze or otherwise bond the two together. If thermally conductive bonding material is used, the bonding material as well as the contact between sheet and tubing will provide a thermal path, and if sufficient filleting of the gap between the curving side of the tube and the sheet is produced, the energy transfer may be quite efficient. Producing maximum transfer efficiency does consume a large amount of bonding material, however, and is at best an uncertain and difficult-to-control process. In addition, use of soldering or brazing requires large amounts of heat to be applied, thereby necessitating that the bonding process take place before insulation is added to the collector. This additional step in processing adds to the handling requirement and therefore to the cost of producing the plates.
It has been proposed to eliminate or reduce the problems of bonding while achieving high thermal efficiency by curving the sheet to conform to the tubing, and previous attempts to secure a close thermal bond by means of forming a conductive copper plate around a copper tube have been described in patents issued to several applicants. In each case the method described involves separate formation of a groove generally conforming to the outline of the tubing which is then placed inside the groove. Various other procedures are used to cause the tubing wall to collapse inward, causing portions of the tubing to expand outward against the backing plate or to cause the backing plate to fold slightly around the tubing. In a patent issued to Sandburg U.S. Pat. No. 2,666,981, for example, the groove is formed in a backing plate by a cooperating roller and die, the roller having a protrusion which forms a groove against a mating die surface. Once the groove is formed, a second step also involving a roller flattens the groove slightly, causing partial collapsing of the side walls from a straight configuration into arcuate configuration. Into the groove is then placed a tubing having a diameter slightly smaller than the width of the groove. In a fourth step, the top of the tube is pressed with a third roller which urges the sides of the tubing outward against the arcuate wall of the groove in the backing plate which is held firmly affixed in a die.
A second patent issued to Sandburg 2,585,043 describes a heat exchanger which is manufactured by first providing a grooved backing plate having raised ridges on each side of the length of the groove. After a tube has been formed and is accommodated into the groove, portions of the raised ridge are deformed by pressing them against a backing die causing the ridges along the groove to overlap the edge of the round tube, thereby entrapping the tube at the bottom of the groove.
In a related method patent, O'dell 1,971,723 describes a method for securing automobile top covers which provides a groove in a metal surface having an arcuate bottom and sides essentially circular and a split tube which fits within the tube. The automobile top cover is placed over the groove and the split tube forced into the groove entrapping the cover between the groove and the metal surface. Then, to secure the top cover permanently, the split tube is compressed forming a flat surface even with the surface of the metal surface and causing the top cover to be compressed against the arcuate sides of the groove. Since greater than 180.degree. of arc is provided, a portion of the metal surface wraps around the compressed split tube thereby retaining it. The split in the tube allows accommodation for movement of the side wall of the tubes without causing deformation of the metal surface.
All of the above-described processes suffer form the common requirement of careful, labor-intensive steps to ensure good thermal bonding. Of those processes which employ some form of groove adapted to receive the copper tubing, all require multiple press operations employing expensive dies. In all cases, the shape of the tubing must be carefully matched to the groove to achieve proper fit. Slight variances in one or the other will result in poor fit and loss of thermal efficiency, or in extreme cases, scraping of the non-fitting parts. The roller die employed in the first Sandberg invention is not amenable to use in forming sinuous patterns in the copper sheet since only straight grooves can apparently be formed.
It is desirable therefore to provide a method for manufacturing solar collector panels which avoids the above problems and which results in a collector panel which is thermally efficient and inexpensive of both labor and materials.